Lead-free solder contains, in addition to tin (Sn) as the main element thereof, copper, silver, zinc, nickel, cobalt, bismuth, indium, phosphorus, germanium, etc. in proper concentrations thereof. Lead-free solder exhibits wetting effect at the melting point thereof or a higher temperature, typically, at 250° C. or higher. In the soldering process, a component such as a printed board is immersed into a lead-free solder bath heated at a temperature within that temperature range, or put into contact with molten solder wave generated in a solder bath.
Copper used onto the printed board or component leads is heated to the above-mentioned temperature range in the soldering process, and dissolves into the molten solder. This is so-called “copper leaching.” When copper leaching takes place, the copper concentration in the solder bath sharply rises, and it leads to a rise in the melting point of solder. Surface tension and fluidity of solder are also affected. As a result, soldering defects such as surface roughness of soldering finish, solder bridge, pits, dry joints, projections, and icicles, are caused, and soldering quality is degraded.
If the copper concentration rises in the solder bath, part or whole of the solder in the solder bath is entirely replaced. Drained solder is dumped or processed to separate excess copper to recycle tin. Recycled tin is re-used as a solder resource material.
Known tin recycling methods include a method of using difference of the melting points, an electrolytic refining method, etc.
The known tin recycling method requires a large-scale facility, and thus a large facility installation area. To keep a material to be refined at a high temperature, flames must be properly handled, and heaters consuming a high electric power, an electrolysis bath, etc. must be required. The installation of these devices leads to environmental problems, exposing workers to dangerous and inefficient jobs there.